Life on Earth: Our Planet would have become habitable sooner than expected

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Zircons are minerals born in the magma that can pass through the billions of years by transmitting the memory of their birth. Measurements of the isotopes they contain suggest that the Hadean Land has formed a continental crust faster than expected, thus helping to create a habitable and inhabited world.

Cosmologists are looking for the first stars and geologists of older terrestrial rocks. These two quests are essential to clarify our understanding of the stages that led from the Big Bang to the living . But if the observable cosmos is somehow “arranged” to keep us light from these first stars, it is very far from the case for the first terrestrial rocks.

More back in the past beyond 3.5 billion years, more geological records are scarce, because they have been almost completely erased by the turbulence of plate tectonics and Wilson’s cycles . It is difficult to decipher those that remain from the beginning of the Archean, the geological era that began 4 billion years ago to finish 2.5 billion years ago. For the geological history that extends beyond -4 billion years, the Hadean (named after the god of Hades Hades) is even worse and, it is believed, for good reason. Indeed, it would have been marked by an intense meteorite bombardment sustaining a long ocean of magma.

Such conditions would of course have been unfit for life and especially for the conservation of rocks of that time. Even imagining an early Earth’s crust with extremely active plate tectonics, due to strong convection currents inside the young Earth heated by the significant disintegration of radioactive elements (and also because of the heat brought by the accretion of the planet), the still intense meteorite bombardment of that time, and the tectonics itself, would have erased almost all traces of the first rocks.

However, for some time now there have been indications that the Earth has become lenient, and perhaps enough to allow the appearance of liquid water in large quantities and even life, some hundreds of millions of years. years before the end of the Hadean. As an article published in the journal  Pnas by American researchers led by members of the University of Chicago, the study of zircons would give any other perspective on the Hadean.

A continental crust in the middle of the Hadean?

To understand this, let us first recall that zircon is a mineral of the group of silicates, more precisely neosilicates, and its chemical formula is ZrSiO 4. Exceptionally resistant to any modification of their chemical composition, zircons appear as one of the early products of the primary crystallization of magmatic rocks, such as granite, and alkaline rocks such as pegmatite or syenite. They are found in sediments in the form of detrital zircons, that is, grains transported and carried away by erosion. They are faithful recorders of the conditions that prevailed during their training. Not only does it cross large periods of time without being altered, but above all, it keeps intact the quantity of isotopes it contained during its formation, with of course the radioactive decay products. As a bonus, we can date their training. Claude Allègre n ‘US geochemists have used a new instrument recently commissioned, the  CHicago Instrument for Laser Ionization (CHILI) to detect isotopes of strontium, and especially measure their abundances (also with isotopes of rubidium), in zircons present in Canadian rocks dating from the Hadean and found in the famous Nuvvuagittuq. It is thought to have found in this province of Quebec, perhaps, the oldest rocks known to date and the oldest traces of the existence of life .

The measured strontium abundances make it possible to go back to the quantity of silica present in the igneous rocks where these zircons were formed. According to the researchers, they suggest the existence earlier than expected of a continental crust, only 350 million years after the birth of the Solar System.

However, the existence of a crust, via its composition, directly affects that of the atmosphere , of the sea water and finally influences the available nutrients for all incipient life forms. In addition, this could indirectly indicate that the meteorite bombardment rate had decreased earlier than expected, allowing the formation and survival of the continental crust.

If that was the case, the Earth became welcoming sooner than expected…

  • Geological records dating back more than 4 billion years are rare and difficult to decipher because of plate tectonics.
  • Zircons are very stable minerals born in magma that can go through the billions of years even when they are no longer in their original rocks while retaining the memory of their formation.
  • Recent measurements of isotope abundances of strontium in zircons found in Canadian rocks suggest that these zircons were part of a continental crust less than 350 million years after the birth of the Earth.
  • The early existence of such a crust also suggests that the Earth had become favorable to the appearance of life earlier than expected during the Hadean.

Life on Earth may have been inevitable

It is the Earth itself that has engendered life, by excess of energy, as surely as the stormproduces lightning. This is the hypothesis put forward by two American scientists, one biologist and the other physicist.

On a young Earth bombarded with meteorites and in an environment very different from ours, fragile molecules have assembled to form living beings. This transformation is still a mystery and many think we had a huge chance. “Not at all,” says Harold Morowitz, a biologist at Fairfax University, Virginia, and Eric Smith, a physicist at the Santa Fe Institute, New Mexico. Life is an inescapable consequence of the geological conditions that prevailed at the time and in particular the accumulation of energy. According to them, life “sprang from this environment in the same way that the lightning discharges their electric charge storm clouds “.

To support their thesis, the two researchers explain themselves … that they have no decisive arguments. But they advance a scenario whose geological and chemical phenomena form the frame. First, volcanic eruptions disperse polyphosphates into the environment. These molecules would have been abundant in the primordial atmosphere whereas they are absent today. On the other hand, they subsist within all living beings where they play a role of energy vectors in cells.

Add chemical reactions between the seawater and the iron that was dissolved and this is the atmosphere that is enriched in hydrogen , rare today but not at the time. The volcanoes, yet them propel huge amounts of carbon dioxide, which does not fail to react with hydrogen to form complex organic molecules.

More stable with life than without

Horowitz and Smith even imagine a cycle of Krebs upside down on a global scale. The Krebs cycle, or citrate , runs continuously in each of our cells and more precisely in their mitochondria to recover energy from organic molecules, for example those we eat, transformed into carbon dioxide. In the young Earth, hot and full of energies of all kinds, it would have turned in the other direction: energy plus carbon dioxide would have led to the formation of organic molecules.

Commenting on these ideas in  Nature, Michael Russel (California Institute of Technology , Pasadena) finds them “informative and inspirational ” and explains that life is a chemical system that channels and dissipates chemical energy.

With this vision, we can imagine that life as we know it must appear on any planet similar to ours, provided there is liquid water and some commonplace ingredients. And let there come a terrible catastrophe, which we call mass extinction , and life goes off again. Because, conclude the two researchers: ”  the Earth is more stable with life than without “.

Dr. Kimberly Seltzer

Postdoctoral Scholar, UC Berkeley Research Assistant, MIT

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